Recent prevention and intervention trial results imply that consumption of antioxidant-rich foods early in disease states provides a protective advantage, while late intervention with dietary carotenoids/retinoids may exacerbates cancer. We conducted a series of mechanism-based hypothesis driven studies designed to investigate the paradoxical results observed in human intervention trials. We hypothesized that increased antioxidant cellular environment (provided by high levels of dietary supplementation with the chemical antioxidant N-acetyl-L-cysteine or NAC), would alter the cellular redox state and promote cancer in carcinogen initiated mice. We have observed that high levels of dietary NAC increases survival and reduces multiplicity of skin malignancies but increases the fraction of malignancies induced by topically administered B[a]P. The majority of the malignancies (keratoacanthomas, squamous carcinomas, and spindle cell tumors) induced in FVB/N-p53 deficient Tg.AC mice demonstrated mutant transgene ras expression and were nucleus positive for p53 (variable intensity from 0 to +4 areas) and treatment independent. Malignancies, both negative and positive for transgene expression were also negative for mutations in codon 12 and codon 61 of endogenous c-Ha-ras protooncogene. NAC alone is mitogenic to splenocytes. In addition, we conducted an in life study (gamma irradiated FVB/N-heterozygous p53+/- mice at 0, 2, 4, or 6 Gy) with and without NAC supplementation in the diet. In this study, we observed a significant decrease in latency for thymic lymphoma and an increased incidence of thymic lymphoma after 24 wks (0/15 vs. 10/15) in the controls versus 4 Gy plus NAC. These results may be explained, at least in part, by in vitro studies that show that lipopolysaccharide (LPS, a B-lymphocyte mitogen) induced splenic cell proliferation (3:1 T- to B-cell ratio) is significantly increased by NAC, but apoptosis was significantly suppressed. Suppression of apoptosis was confirmed by independent assays (TUNEL and Annexin V). Lymphocyte origin specificity was confirmed by flow cytometry using B- and T-lymphocyte specific monoclonal antibodies. We are currently investigating the molecular mechanisms involved in pro-oxidant exacerbation of lymphomagenesis. These studies include changes in gene expression at the mRNA and protein level as well as molecular genetic changes in signaling pathways critical to proliferation and apoptosis. In addition, additional in vitro studies have focused on 1) mechanism of NAC associated suppression of apoptosis using microarray based gene expression, followed by confirmation using protein 2D gel electrophoresis and western analysis (where required for confirming dysregulated pathways).